The invention relates to a method for coating an optoelectronic chip-on-board module, comprising a flat substrate populated with one or more optoelectronic components, with a transparent, UV-resistant, and temperature-resistant coating made of a silicone, a corresponding optoelectronic chip-on-board module, and a system having multiple optoelectronic chip-on-board modules.
Generic optoelectronic chip-on-board modules are used, for example, as illuminating bodies, as high-power UV LED lamps, as photovoltaic modules, as sensors, or as similar devices. Within the scope of the invention optoelectronic components used in this way include, for example but not exclusively, LEDs or photodiodes in the form of chips or other components arranged in the chip-on-board module on a flat substrate, that is, a metal, ceramic, or silicon substrate, a metal core or FR4 circuit board, a glass substrate, a plastic substrate, or a similar device. These chip-on-board modules must be protected from mechanical damage and corrosion. For this purpose the most compact and lightweight solutions are desired.
From the prior art, light-emitting diode modules having a light-emitting diode array and a micro-lens array are known in various configurations. Examples of such light-emitting diode modules and methods for their production are described, for example, in U.S. Pat. No. 7,638,808 B2, in U.S. patent application publication 2010/0065983 A1, in US 20070045761 A1, or in U.S. Pat. No. 7,819,550 B2. In German published Patent Application No. DE 10 2010 044 470, a method for producing a micro-lens array for an optoelectronic module is also described.
Protection in the form of housings on chip-on-board modules is often expensive and technologically complicated. A practical alternative to the protection of chip-on-board modules is a flat encapsulation of the components with a plastic-based casting material. Together with other functional components, for example strip conductors and contacting elements, the optoelectronic components in chip-on-board modules together with a flat substrate are protected from mechanical damage and corrosion by coatings.
For this purpose, injection molding methods or casting methods with epoxy resins are typically used. These resins are initially applied as liquid casting material and then cured by heat and/or radiation. Because the casting material is initially liquid, the casting material must be prevented from flowing away. This is typically realized by a mold or a fixed frame.
One alternative to these methods is the so-called “dam-and-fill” method, wherein a plastic dam that surrounds an area of the substrate is initially deposited on the substrate of the chip-on-board module. A liquid filling material made of epoxy resin is then filled into this area. This filling material is cured. The dam and filling material together form the coating of the module. For generating the dam, a viscous polymer is applied or drawn with a dispensing device in this method and then cured, so that casting material can be cast onto the area enclosed by the dam, without this casting material flowing away.
The plastic dam generated in this way, however, is not transparent. Therefore, this method of coating negatively affects the light emission intensity or light sensitivity of optoelectronic chip-on-board modules, that is, chip-on-board modules populated at their edges with optoelectronic components, for example LEDs or photodiodes.
These mentioned methods using epoxy resins are less suitable for optoelectronic use, because epoxy resins are not UV-resistant and temperature-resistant. Thus, they are not stable, for example, in a high-power UV-LED module or also under intense sunlight irradiation having UV components, as can occur in photovoltaic systems. They age quickly under UV exposure and are destroyed.
Other transparent, UV-resistant, and temperature-resistant solutions, for example bonding a glass frame or a glass cup, require a very complicated assembly of the frame and a compactness of the frame that can be produced only with difficulty. Such a solution is also associated with a greater weight than a casting solution. For rigid glass materials, a usually necessary adaptation of the thermal expansion coefficients of the compound materials also represents another hurdle, especially if the resulting products are exposed to thermal cycles.
In a combination solution made of a glass frame and an encapsulation with a suitable non-epoxy-based material, e.g., a temperature-resistant and UV-resistant silicone, very small gaps between the frame and substrate can have the result that the silicone, which is strongly susceptible to creep, can run out during the casting. Space for the frame must also be provided on the substrate. This negatively affects a best possible utilization of the substrate surface area and/or a desired stackability.
Due to the necessity for preventing the liquid casting material from flowing away, known injection molding methods and casting methods require a vacuum seal that seals the edge of the module. This results in a loss of usable surface area on the module, because the edge area must be kept free from components.
Up until now, no method for realizing a flat coating for chip-on-board modules has been known in which materials can be used both in the face area and also in the edge area of the casting, which materials are both UV-resistant and also temperature-resistant and that are also transparent for electromagnetic radiation from the ultraviolet to the infrared spectral range.
In European patent application publication EP 1 657 758 A2, a corresponding casting method for LED units on a support structure for generating lenses for the LED units is known in which liquid silicone is filled into a negative mold for the lens structure and the support structure is placed with the LEDs on the mold, so that the LEDs are immersed in the liquid lens material. Around the support structure of the module and the mold, a vacuum seal is applied that acts at the edge of the support structure or the substrate of the module and the mold and presses the two together under a high pressure, in order to prevent the liquid material from coming out. In this way, lenses are shaped around the LEDs, while the surface area between the LEDs remains essentially unwetted from casting material up to creeping material.
With this method it is not possible to provide optoelectronic chip-on-board modules with a coating that is transparent, UV-resistant, and temperature-resistant and that can be populated arbitrarily with optoelectronic components, especially while utilizing the entire surface area.